The present invention relates to blocked isocyanates and methods for making the same. More particularly, the present invention relates to methods for making blocked isocyanates which utilize a more effective catalyst than was available in the prior art. Blocked isocyanates prepared by the method of the present invention contain substantially less undesired byproducts in the reaction product than was heretofore possible.
Organic polyisocyanates and especially the diisocyanates are commercially important both as starting materials for a number of resins and as crosslinking agents for polymer systems having reactive hydrogen-containing groups; e.g. alcohols, aldehydes, amines, amides and the like. When isocyanates are to be used as crosslinking agents, the highly reactive isocyanate groups are frequently deactived by blocking until crosslinking is to be effected. Blocked isocyanates can be prepared by reacting the isocyanate moiety with a monofunctional compound such as an alcohol to form the corresponding urethane. At high temperatures, for example, in a wire tower, the urethane unblocks, liberating the starting alcohol and regenerating the free isocyanate groups which can be used to crosslink the wire enamel polymer system.
Bunge et al., U.S. Pat. No. 2,952,665, discloses that cyclic trimers of diisocyanates containing three blocked isocyanato groups can be prepared by reacting an organic diisocyanate with a monohydroxy compound and thereafter adding a tertiary amine catalyst at temperatures of at least about 150.degree. C. Representative tertiary amines suitable for use as a catalyst include dimethyl aniline, hexahydrodimethylaniline, 4-methyl morpholine, 4-ethyl morpholine, permethylated diethylene triamine or triethylene tetramine, urethanes consisting of 1 mol of N,N-dialkylaminoethanol and 1 mol of phenyl isocyanate or cyclohexyl isocyanate, N-dialkyl piperazine and N,N'-dimethylethanolamine. Consideration of the examples reveals that anywhere from 5 to 25 hours of heating at temperatures of at least 150.degree. C. is necessary, depending upon the particular reactants, the catalyst and the amount of catalyst employed to obtain a blocked isocyanate.
It has now been discovered that amines having the general formula ##STR2## where each R is independently selected from hydrogen and lower alkyl radicals and R.sup.1 is an alkylene radical having up to about six carbon atoms, are more effective as catalysts than any of the amines disclosed by the prior art. Tetramethylethylenediamine has been found to be especially effective as a catalyst. Quite unexpectedly, the use of tetraalkylated alkylene diamines of Formula I allows the artisan to effect trimerization of isocyanato molecules at much lower temperatures in a comparable amount of time, or at elevated temperatures, such as employed by Bunge et al., in a substantially shorter period of time. It has also surprisingly been found that compositions prepared according to the present method, especially at temperatures less than about 120.degree. C., contain less undesired by-products than obtained in prior art methods.